When a wiring board is produced, there has been generally employed a process which includes using an insulating substrate such as a resin plate having a through hole, dipping the insulating substrate in a plating solution, and then carrying out electroplating to fill the through hole with a metal (see, for example, Patent Literature 1 and Patent Literature 2). However, when the above-mentioned process is employed, there is a possibility to generate an air gap which is a so-called void in a metal layer which is filled in the through hole of the insulating substrate by electroplating. Therefore, the process has a defect such that reliability of the wiring board is remarkably lowered.
As a process for producing a wiring board, which hardly generates a void in a metal layer filled in a through hole, there has been proposed a process for producing a wiring board, which includes using an insulating substrate having a through hole, forming a seed layer on one surface of the insulating substrate, covering the surface of the insulating substrate opposite to the surface on which the seed layer is formed with a masking film, arranging the insulating substrate and a positive electrode so that the surface of the insulating substrate on which the seed layer is formed is faced to the positive electrode, carrying out electroplating to close the through hole existing on the surface on which the seed layer is formed with a metal layer, thereafter removing the masking film from the insulating substrate, arranging the insulating substrate and a positive electrode so that the surface of the insulating substrate from which the masking film has been removed is faced to a positive electrode, carrying out electroplating to form a metal layer in the through hole (see, for example, Patent Literature 3).
According to the above-mentioned process for producing a wiring board, a wiring board having high reliability can be produced because a void is hardly generated in a metal layer filled in the through hole.
However, in recent years, it has been desired to develop a process for producing a wiring board, in which not only a void is hardly generated in a metal layer filled in a through hole, but also a wiring board can be efficiently produced without a process for closing a through hole existing on the surface of the wiring board on which a seed layer is formed with a metal layer.
Furthermore, in recent years, it has been desired to develop a process for producing a wiring board, which enables to shorten a period of time necessary for plating, and enables to reduce the amount of a metal by reducing the thickness of a metal layer formed by electroplating on the surface of a seed layer, and which also enables to make an electrode smooth.